iidx,sdvx: disable subscreen overlay if -monitor is in use (#471)

## Link to GitHub Issue, if one exists
#345 

## Description of change
If TDJ / VM mode is active AND user is enabled `-monitor` option (and
the value isn't 0, the primary monitor)

then disable the subscreen overlay, show an error message instead.

We're doing this because both games cannot accept emulated touch input
if `-monitor` option is in use. There is not much point in displaying
the second screen's image, other than to REALLY confuse people when
nothing happens when they click on it.

## Testing
Tested both games with `-monitor`.

src/spice2x/games/sdvx/sdvx.h

@@ -23,6 +23,9 @@ namespace games::sdvx {
     extern bool BI2X_INITIALIZED;
     extern SdvxOverlayPosition OVERLAY_POS;
 
+    // states
+    extern bool SHOW_VM_MONITOR_WARNING;
+
     class SDVXGame : public games::Game {
     public:
         SDVXGame();
@@ -30,5 +33,11 @@ namespace games::sdvx {
         virtual void attach() override;
         virtual void post_attach() override;
         virtual void detach() override;
+    private:
+        // don't be tempted to make this into a public or a global variable
+        // various modules need to check if VM mode is active and they should
+        // not depend on the fact that SDVX module is active (since it can be
+        // inactive on cabs or partial I/O setup)
+        bool VALKYRIE_MODEL = false;
     };
 }